It is known that properties of a film, coating, or layer of a given material differ from those of the same material in bulk form. For example, thin films have different yield stresses, creep behavior, and elastic properties than do bulk forms of the same material. The only way to determine the qualities or properties of thin films is to measure the films themselves.
As technology advances, many elements including storage media (e.g. disks), integrated circuits, cutting tools, sensor arrays, wear surfaces, LCD matrix arrays, and the like include films, coatings, and/or layers deposited on a substrate. The term "film" as used herein is to encompass films, coatings, and layers of varying thicknesses. Typically, each film (e.g. thin film) has different residual stress characteristics and different thermal and/or mechanical properties, which may affect the performance, reliability, or durability of devices including such films. The ability to determine mechanical properties and residual stress characteristics of thin films is thus desired. Bulge testing is one way in which to do this.
In prior art bulge testing systems, as shown in FIG. 1, circular or rectangular film 1 having a thickness "t" is clamped over cavity or orifice 3 in mounting structure 5, and pressure is applied to the bottom side of film 1 from within the orifice. The out-of-plane deflection or bulging of film 1 is measured as a function of the applied pressure enabling determination of a pressure-deflection curve and the residual stress in the film. Prior art FIG. 2 shows the vertical equilibrium of film 1 when pressure is applied via cavity 3.
The stress state of film 1 is two dimensional so that properties in the plane of film 1 are measured through the use of known equations which include as parameter(s) the geometry of the film, the properties of the material composing the film, the differential pressure applied across the film, the center deflection of the film, and in-plane residual stress. For example, see "Mechanical Properties of Thin Films" by Nix, found in the 1988 Institute of Metals Lecture, Volume 20A, November 1989; "Measuring the Mechanical Properties of Thin Metal Films by Means of Bulge Testing of Micromachined Windows" by Paviot, et. al., Mat. Res. Soc. Symp. Proc. Vol. 356, 1995 Materials Research Society; "Mechanical Behavior of Thin Films" by Vinci and Vlassak, Annu. Rev. Mater. Sci. 1996-26:431-62; "The In-Situ Measurement of Mechanical Properties of Multi-Layer Coating" by Lin, 1990 MIT Dept. of Mat. Sci. & Eng., Archives; "Load Deflection Analysis for Determining Mechanical Properties of Thin Films With Tensile and Compressive Residual Stresses" by Bulsara, 1995 MIT Dept. Mat'l. Sci. & Eng.; and "New Experimental Techniques and Analysis Methods for the Study of the Mech. Prop. of Materials in Small Volumes", Chapt. 3, by Vlassak (1994), the disclosures of which are all hereby incorporated herein by reference.
Bulge testing of circular or square freestanding windows of different geometries of film 1 mounted to structure 5 allows one to determine the biaxial modulus of the film as well as the residual stress in the film. Knowledge of these characteristics is important in determining durability and other mechanical and structural characteristics of the film.
With regard to square films or membranes, for example, the elastic deflection as a result of a uniform pressure "p" applied in the cavity is known to be approximately described by the following equation (see Paviot, et. al. referenced above): ##EQU1## where c.sub.1 (v) is about 1/(0.792+0.085 v).sup.3 and c.sub.2 equals about 3.393. In this expression, w.sub.0 is the deflection of the center of the film or membrane, "t" is the film or membrane thickness, and "a" is the width of the membrane. Utilizing the above-identified equation enables one to determine the biaxial modulus Y=E/(1-v) and the residual stress in the film.
As disclosed in Vinci and Vlassak (cited above), the pressure-deflection relationship for a thin circular film or membrane with a residual stress in a bulge test is approximated by the equation: ##EQU2## in the elastic regime, where w.sub.0 is the deflection of the center of the film or membrane, "P" is the applied pressure, "t" is the film or membrane thickness, and "a" is the film or membrane radius. Using this equation enables one to determine the biaxial modulus E/(1-v) and the residual stress in the film.
It is noted that other equations, which are disclosed and explained in the above-identified publications, may be used to determine residual stress and/or elastic modulus of films subjected to bulge testing.
It is also known to test composite membranes including two or more layers. For example, see pages 90+ in Chapter 3 of Vlassak, "New Experimental Techniques and Analysis Methods for the Study of the Mechanical Properties of Materials in Small Volumes" (1994), where bulge testing of a composite membrane including two or more layers is discussed. As discussed by Vlassak, silicon oxide or silicon nitride films can be used as substrates or membranes onto which metal films are deposited. This technique can be applied to a variety of films without major changes to the sample preparation method.
Still referring to Chapter 3 of Vlassak, pages 90+, it is known that the residual stress in, and elastic properties of, the silicon nitride or silicon oxide membrane by itself can be determined by bulge testing the membrane without a film overlayer. Thereafter, when a metal overlayer film is deposited onto the silicon oxide or silicon nitride membrane, its biaxial modulus can be calculated from the biaxial modulus of the composite film. If Poisson's ratio of the metal film is known, Young's modulus of the metal film can be calculated from the biaxial or plane-strain modulus. The residual stress in the metal film is calculated via the average residual stress in the composite, as the weighted average of the stresses in the membrane and the metal film overlayer.
As discussed in section 3.4 of Vlassak, Chapter 3, it is known to fabricate freestanding silicon nitride films on silicon substrates by way of micromachining. Such silicon nitride membranes are then used as substrates or membranes for other films (e.g. metal films) and the resulting composite film is bulge tested. Referring to prior art FIGS. 3(a)-3(f), known steps are shown in a sample preparation process. As illustrated in FIGS. 3(a) and 3(b), silicon nitride films 7 with residual tensile stress are deposited by LPCVD on both sides of wafer 9. Using mask 11 illustrated in FIG. 3(c), a window is etched in silicon nitride film 7 on the backside of the wafer by way of lithography and reactive ion/plasma etching. The etched window is illustrated in FIG. 3(d). Thereafter, as shown in FIG. 3(e), silicon 9 exposed by the previously etched window is etched using, for example, an etchant including potassium hydroxide. FIG. 3(e) illustrates the final silicon membrane wafer structure with a freestanding flexible silicon nitride membrane over the cavity on its top surface. The freestanding flexible membrane portion of layer 7 in FIGS. 3(e)-3(f) is defined within the silicon shoulder area, where film 7 is susceptible to bulging. Finally, as illustrated in FIG. 3(f), a thin metal film 13 to be bulge tested is evaporated onto the top surface of the membrane structure.
After the FIG. 3(f) structure is made, as disclosed by Vlassak, it is bulge tested using the prior art FIG. 4 apparatus, which includes mounting structure 15 upon which composite sample member 17 to be bulge tested is mounted, pump 19, pressure gauge 21, computer and data acquisition terminal 23, and an inspection system for detecting deflection of film 17. The inspection system including laser 25, beamsplitter 27, collimator 29, lens 31, reflective mirror 33, density filter 35, reference mirror 37, and screen 39 with an interference pattern. As described by Vlassak, sample 17 to be tested is glued onto mounting structure 15 and pressure is applied to the lower side of sample 17 by pumping water into cavity 41 via pump 19. The inspection system then measures the deflection of sample 17 caused by the water pressure in the cavity. The result is a pressure versus deflection plot for the sample. From this plot, the elastic modulus and residual stress of overlying film 13 can be determined.
U.S. Pat. No. 4,735,092 to Kenny, discloses a rupture testing apparatus for classifying or grading metal foils. Gas under pressure is admitted to a platen and the unsupported part of the sample bulges outwardly until the sample ruptures. A plot is made of samples for temperature, burst pressure, and bulge height at burst, with the results being used to grade or classify the foil. Unfortunately, the '092 patent suffers from a number of problems, including the inability to efficiently and properly determine stress and/or modulus characteristics of the film being bulge tested. For example, the dial micrometer transducer includes a probe or arm which extends downward to contact the film being bulge tested. Contacting type transducers are generally undesirable, especially in view of the fragile nature of many samples that must be tested. Further deficiencies in the '092 system are discussed below.
While the above-referenced prior art bulge testing techniques and disclosures achieve satisfactory results in non-commercial environments where cost and efficiency are not critical considerations, they unfortunately have their limitations. A few of these limitations are discussed below.
The characteristics and properties of films, coating, and layers used in electronic arrays, wear surfaces, circuits, cutting tools, and the like are becoming more and more important. Different systems and techniques are utilized to deposit and/or pattern thin films on substrates. For example, a uniform thin film indium tin oxide (ITO) layer a few hundred .ANG. thick may be deposited across an entire substrate, and thereafter sometimes patterned via conventional methods into a plurality of electrode segments. Due to the techniques and systems used to deposit and/or pattern such thin films, it is not surprising that areas of the thin film(s) on the substrate, or certain patterned electrodes in the array, may have different residual stress and modulus characteristics than others. For example, film near an edge of the substrate may have different stress characteristics than near the center of the substrate, due to the techniques and systems utilized in the deposition, fabrication, and/or patterning. For example, a continuous film may have different residual stress and/or elastic modulus characteristics in different areas on the substrate. Differences such as these in large area substrate or array-type applications cannot be detected by the prior art bulge testing systems discussed above.
Bulge testing has been minimally successful at best, for reasons such as high substrate/membrane costs, the inability to commercially manufacture scores of reproducible substrates/membranes within limited predetermined dimensional and compositional tolerances (i.e. very difficult and expensive to make substrates of constant dimensions which all have the same characteristics), inaccurate substrates/membranes, inability to test large area films, and the like.
There also exists a need in the art for reproducible circular, rather than square, membranes to allow analytical equations to be used to calculate thin film mechanical properties from pressure-deflection data rather than having to use numerical methods. Also, a need exists for membrane material having reproducible mechanical properties in contrast to currently produced silicon nitride or silicon oxide whose mechanical properties vary as a function of deposition/growth parameters, and equipment used to manufacture same.
It is apparent from the above that there exists a need in the art for a bulge testing system that can be utilized to test large area thin films (or portions thereof) on substrates, and thin film segments as they are deposited in array form on a substrate. There is also a need in the art for a way in which to fabricate supporting substrates/membranes so that on a continuous basis all such supporting membranes are fairly identical, with their geometries, mechanical responses, and/or material properties being substantially the same. Therefore, in a commercial bulge testing environment, it would be desirable if there were no need to separately bulge test each membrane structure and determine its characteristics prior to applying thereto a thin film to be tested. The ability to mass produce many such uniform supporting membranes would result in increased efficiency and significant cost savings in commercial testing environments. There also exists a need in the art for an improved membrane structure for supporting thin films to be bulge tested. Other needs include the need for precision mounting of membranes, automated measurement, improved deflection detection techniques, and improved software for manipulating the table or platform upon which the membrane structure is mounted.
It is a purpose of this invention to fulfill the above-described needs in the art, as well as other needs which will become apparent to the skilled artisan from the following detailed description of this invention.